Should I continue to let the 3 leaders thicken, or chop and start to build direction and taper? Should I keep all 3 or remove one?
First off, quit with the wire. Wire is mostly for pines and secondary branching on deciduous trees. It does little good (and potentially a lot of harm) on bigger branches on deciduous trees. Branching on deciduous trees is best accomplished with "clip and grow" to build branches--this is ESPECIALLY true with oaks. Wiring creates rounded, smooth curves on thicker deciduous shoots--that look strange and unconvincing.
You make a bonsai out of an oak to capture a rugged, aged appearance--which is why you collected the trunk in the first place. Smooth curves are not dramatic and don't fit.
I don't think this is going to work very well for a "broomish" tree. The top branching is developing into twin, uninteresting apexes. The transition between the branches and the trunk is weird and probably isn't going to get any better....
I would choose the right hand shoot as the "grow ahead" apex branch. I would eliminate ALL of the others at this point. I'd allow the remaining shoot to extend with no intervention for another three or four year to begin to match the main trunk's diameter. When that is done, I'd chop it back to two inches and wait for buds to pop on the trunk--During the grow out period, you can also hope for shoots farther back down the trunk. If one or two or three appear, they could be the start of branching, but they have to be reasonably placed-not from the main chop site...
When and if those buds appear lower down, encourage them to grow OUT, not up. That means gently manipulating the shoot every so often into a more lateral position. Oaks tend to push growth UP and not out. You have to push it flatter.
This is a long term project--like 5-10 years. It will require patience.